Rewrite copier code to improve symlink handling and preserving metadata

[felixfontein]

This is inspired by #7 (comment).

Symlink handling:

• Symlinks inside the tree are kept as symlinks. (Thus dead symlinks are kept.)
• Symlinks outside the tree are copied by content. (Dead symlinks to outside are causing errors.)

This is closer to what ansible-galaxy collection build does, except that its outside detection is not correct IMO (the one here should be better). More precisely, as an example, a symlink pointing to ../<collection_directory_name>/galaxy.yml is treated as an outside link by this code, while it is treated as an inside link by ansible-core's code (it computes the absolute path of the destination and checks whether it is prefixed by the absolute path of the collection directory, which in this case would be true). (See below.)

[felixfontein]

Ok, I checked the ansible-core code a bit more: the ../<collection_directory_name>/galaxy.yml symlink is actually changed to a symlink to galaxy.yml, i.e. symlinks are normalized.

I guess we should also do that, then.